Copending, commonly assigned U.S. patent application Ser. No. 071,509, now U.S. Pat. No. 4,295,741, incorporated herein by reference, discloses a method of eliminating the 2.pi.N ambiguity in a composite beam composed of a number of beams from a set of commonly controlled power amplifiers which receive and amplify portions of a master oscillator beam. A total combined output beam having a uniform phase front can be formed by combining the output beams in such a manner that all path lengths from the master oscillator through the several amplifiers are equal. If the path lengths differ by an integral number of wavelengths, the corresponding beams will differ in phase by 2.pi.N radians. In the case of lasers that generate radiation over a frequency range that includes two or more spectral lines, a beam-front phase difference of 2.pi.N radians of a fundamental or reference line implies that the associated radiation from different lines will be out of phase by an amount that is not an integral multiple of 2.pi.. In other words, only if all path lengths through the system are equal within one wavelength will the output beam have a uniform phase front at all frequencies of the output radiation.
Copending application Ser. No. 071,509 discloses a method of heterodyne interferometry that employs two selected lines of the beam. Adjusting the path lengths through different one-pass power amplifiers so that radiation from two selected lines are in phase at the output ensures that all lines will be in phase. In the path length control system, error discriminants are formed for coarse and fine control, both of which have sinusoidal dependence on path length mismatch.